Light emitting diode driver providing current and power control

ABSTRACT

As apparatus for controlling operating parameters for a light emitting diode (LED). A driver receives an input voltage from a voltage source and a control signal. The driver provides a driver output current to the LED based on the control signal. The LED has an LED voltage as a function of the driver output current and has an LED power as a function of said LED voltage and said driver output current. A controller determines the LED voltage and produces the control signal provided to the driver as a function of the LED voltage. The control signal corresponds to a constant driver output current when the LED voltage is less than a predetermined voltage value. The control signal corresponds to a varying driver output current for substantially maintaining the LED power at a constant predetermined power value web the LED voltage is greater than the predetermined voltage value.

BACKGROUND

This invention relates to solid-state light sources and particularly tolight emitting diodes (LEDs). More particularly, the present inventionrelates to an apparatus for controlling power and current delivered tothe LED.

Light emitting diodes (LEDs) are semiconductor devices that generatelight when electrical energy (e.g., current, voltage) is applied to thedevice. A driver (i.e., one or more electronic components electricallyconnected to an LED) may be used for selectively applying electricalenergy from an electrical energy source to the LED. A conventional LEDdriver has a particular topology for use with the electrical energysource to provide constant current to the LED. Thus, the LED driverallows the LED to continuously operate at a constant current level. Thetopology for the driver may be buck, boost, or a combination buck andboost (hereinafter referred to as “boost buck”), and is selected basedon the electrical energy available from the electrical energy source andthe electrical characteristics of the LED (e.g., forward voltage). Forexample, an LED driver having a boost buck topology must be used todrive an LED wherein the forward voltage of the LED may overlap theavailable source voltage.

Not all of the electrical energy applied to the LED is converted tolight. A substantial portion of the applied electrical energy isdissipated in the form of heat by the LED. As the semiconductor materialheats up, like most electronics devices, the LED performance isdegraded. In particular, the power (e.g., heat) dissipated by the LEDcan cause decreased light output (flux), a color shift, and a reductionin device lifetime. To minimize the adverse effects of the powerdissipation, various thermal management systems may be incorporated orused in conjunction with the LED.

For an LED driven from a driver having a constant current boost bucktopology the power dissipation of the LED and hence the driver can vary,depending on the forward voltage of the LED. Conventionally, a thermalmanagement system which assumes the worst case power dissipation isemployed to avoid an excessive temperature rise resulting from thevariable power dissipation of the LED. Referring to FIG. 1, for example,the driver may provide a constant current (I_(led)) to the LED until theforward voltage (V_(f)) of the LED reaches a maximum value(V_(led max)). When this condition is met, the driver shuts down orenters into a fail safe mode (i.e., ceases to apply current to the LED).However, this type of thermal management system fails to effectivelysolve the problem. In particular, the system can cause oversized heatsinks and/or require active cooling techniques.

FIG. 2 illustrates another technique for minimizing the adverse effectsof variable power dissipation used for applications other than drivingLEDs. As illustrated, the driver provides electrical energy to the loadsuch that the load operates at a constant power and for a particularrange of current and voltage values. Thus, the driver may providecurrent less than a maximum value (I_(led max)), as a function of theconstant power value, until the voltage of the load reaches a maximumvalue (V_(led max)). When this condition is met, the driver shuts downor enters into a fail safe mode (i.e., ceases to apply current to theload). On the other hand, as the forward voltage decreases, to maintaina constant power level, the controller increases the operating current.Since the operating current needs to be limited to protect the LEDs, thedriver needs to shut down upon reaching the maximum current,I_(led max), which limits the operating region of the system

SUMMARY

Embodiments of the invention overcome one or more deficiencies ofconventional practices related to maximizing LED performance bycontrolling power and current provided to the LED according topredetermined values. In particular, the present invention senses anoperating parameter of the LED and provides either a constant power or aconstant current to the LED based on the sensed operating parameter. Assuch the present invention advantageously minimizes the adverse effectsof power dissipation.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

Other features will be in part apparent and in part pointed outhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating a constant current operating path for anLED with current along the y axis and voltage along the x axis,according to related art.

FIG. 2 is a graph illustrating a constant power operating path for anLED with current along the y axis and voltage along the x axis,according to related art.

FIG. 3 is a graph illustrating an operating path for an LED with currentalong the y axis and voltage along the x axis, according to anembodiment of the present invention.

FIG. 4 is a graph illustrating an operating path with exemplaryoperating parameter values for an LED with current along the y axis andvoltage along the x axis according to an embodiment of the presentinvention.

FIG. 5 is a block diagram illustrating an apparatus for use with anelectrical energy source for energizing a light emitting diode accordingto an embodiment of the invention.

FIG. 6 is a block diagram illustrating an apparatus for use with avoltage source for energizing a light emitting diode according to anembodiment of the invention.

FIG. 7 is a circuit diagram illustrating electrical components of adriver according to an embodiment of the invention.

FIG. 8 is a flow diagram illustrating operations performed by acontroller according to an embodiment of the invention.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

Embodiments of the invention include an apparatus (hereinafter referredto as the “control system”) for use with an electrical energy source forenergizing a light source, such as a light emitting diode (LED).According to embodiments of the invention, the control system provideselectrical energy to the light source as a function of a plurality ofoperating parameters (e.g., current, voltage, power) of the lightsource. In particular, the control system senses the operatingparameters of the light source and operates in a first mode or a secondmode based on one or more of the sensed operating parameters. In thefirst mode, the control system provides electrical energy to the lightsource to maintain (e.g., substantially maintain) a first operatingparameter at a constant value. In the second mode, the control systemprovides electrical energy to the light source to maintain (e.g.,substantially maintain) a second operating parameter at a constantvalue.

Referring to FIG. 3, in one embodiment of the present invention, thecontrol system senses the voltage (V_(led)) of the light source. Thecontrol system operates in a first mode and a second mode as a functionof the sensed voltage (V_(led)). For example, as illustrated by FIG. 3,the system may operate in the first mode when the sensed voltage(V_(led)) is less than (i.e., not greater than) a predetermined voltagevalue and may operate the second mode when the sensed voltage (V_(led))is greater than the predetermined voltage value. In the first mode, thecontrol system provides electrical energy to the light source tomaintain the current (I_(led)) of the light source at a constant value(C_(I)). In the second mode, the control system provides electricalenergy to the light source to maintain the power (P_(led)) of the lightsource at a constant value (C_(P)). FIG. 4 illustrates a specificoperating path and operating parameter values for an exemplaryembodiment of the present invention.

Referring to FIG. 5, in one embodiment of the present invention, thecontrol system 502 is electrically connected to the electrical energysource 504 for receiving electrical energy from the energy source as aninput signal 506. The electrical energy source 504 may comprise acurrent source and/or a voltage source. Correspondingly, the inputsignal 506 may comprise a current signal and/or a voltage signal. Thecontrol system 502 is also electrically connected to the light emittingdiode (LED) 508 for energizing the LED 508, via an output signal 510, asa function of the input signal 506. The LED 508 may comprise one or moreof the following: a light emitting diode, an ultra-violet (UV) emittingLED, an infrared (IR) emitting diode, and a laser diode.

The control system 502 of the illustrated embodiment includes a driver512 and a controller 514 (e.g., microcontroller, programmable logicdevice, processor, microprocessor, computing device) in electricalcommunication with each other. The driver 512 receives the input signal506 from the electrical energy source 504 and a control signal 516 fromthe controller 514. The driver 512, having a particular topology,converts the input signal 506 and provides the converted input signal(i.e., the output signal 510) to the LED 508 based on the control signal516. In an embodiment, the driver 512 receives information to vary theLED current from control signal 516. The output signal defines theoperating parameters (e.g., current, voltage, power) associated with theLED. In particular, the output signal provides the LED with a currentand a voltage (i.e., I_(led) and V_(led)). As known in the art, thecurrent and voltage have a dependent functional relationship.Additionally, the output signal provides the LED with a power (i.e.P_(led)). As known in the art, the power is functionally dependent onthe current and voltage. The controller 514 produces the control signal516. In particular, the controller 514 receives a feedback signal 518representing the operating parameter, V_(led) of the LED and producesthe control signal 516 provided to the driver as a function of thereceived operating parameter value.

Referring to FIG. 6, in an embodiment of the invention, the controlsystem 502 is used with a voltage source 504 for energizing the LED 508.The driver 512 comprises a voltage controlled current source (VCCS).Accordingly, the driver 512 receives a driver input voltage (V_(in)) 506from the voltage source 504 and a control signal 516 from the controller514 and provides a driver output current (I_(led)) to the LED 508 basedon the control signal 516. The LED 508 receives the driver outputcurrent (I_(led)) and has a voltage (V_(led)) as a function of thedriver output current (I_(led)) and a power (P_(led)) as a function ofthe voltage (V_(led)) and the driver output current (I_(led)). Thecontroller 514 determines the LED 508 current (I_(led)). In anembodiment, the control system 502 may further include variouselectrical components, such as an inverting amplifier 602 and/or ananalog to digital converter (not illustrated), for determining the LEDvoltage (V_(led)). The controller 514 produces the control signal 516 asa function of the LED voltage (V_(led)). The controller 514 isconfigured such that the control signal 516 corresponds to a constantoutput current when the LED voltage (V_(led)) is less than apredetermined voltage value and to a varying driver output current forsubstantially maintaining the LED power at a constant predeterminedpower value (i.e. P_(led)=constant) when the LED 508 voltage is greaterthan the predetermined voltage. The controller 514 transmits the controlsignal (e.g., a voltage signal or a current signal) 516 to the driver512. In an embodiment, the control system 502 may further includevarious electrical components such as a low pass filter 604 and anamplifier 606 for transmitting the control signal 516 to the driver 512.In an embodiment, the control system 502 further includes a resetcomponent 608 for resetting the control system 502.

Referring to FIG. 7, in an embodiment of the invention the driver 512has a boost buck topology. Accordingly, the driver 512 includes aswitching component S having a switching frequency for adjusting thedriver 512 output current. For example, the driver 512 may include aswitching transistor S having a duty cycle which is adjusted to providethe driver 512 output current to the LED 508 according to the controlsignal. In an embodiment of the invention, the driver 512 may furtherinclude an integrated circuit (IC) for receiving the output drivercurrent (I_(led)) and the control signal 516 and controlling/adjustingthe driver output signal (I_(led)) based on said received signals. FIG.7 illustrates an exemplary driver 512 comprising a variable frequencypulse width modulation controller integrated circuit IC and having aboost buck topology (e.g., HV9930 hysteric boost-buck LED driver IC)according to an embodiment of the invention. Briefly described, theillustrated exemplary driver 512 includes electrical components L₁, S,C₁, L₂, D₂, and C₂ for performing boost buck functions. The illustratedexemplary driver 512 includes electrical components R_(d) and C_(d) forperforming damping functions. The illustrated exemplary driver 512includes a diode D₁ for preventing electrical discharge from C_(d) andC₁ when switch S is open. The illustrated exemplary driver 512 includesresistors R₁ for sensing a driver input current and includes resistorsRo for sensing a driver output current (I_(led)). The illustratedexemplary driver 512 includes the IC for receiving the sensed driverinput current, the sensed driver output current (I_(led)), and thecontrol signal 516 and adjusting the duty cycle of the switch S based onsaid received signals.

In an embodiment of the invention, the controller 514 includes a storagemedium for storing data for defining the control signal 516 provided tothe driver 512 as a function of the LED 508 operating parameters. Forexample, the storage medium may store a maximum power value, a maximumvoltage value, and/or a maximum current value. Additionally, the storagemedium may include a mapping component for mapping the LED voltage value(V_(led)) received by the controller 514 to values of the driver outputcurrent corresponding to a constant driver output current signal (e.g.,the maximum current value) or a constant LED power (e.g., the maximumpower value). The storage media may be internal or external to thecontroller 514. Exemplary internal storage media include RAM, ROM,EEPROM, flash memory and/or other internal storage media known in theart. Exemplary external storage media include memory sticks, CD-ROM,digital versatile disks (DVD), magnetic cassettes, magnetic tape,magnetic disks and/or other storage media known in the art.

FIG. 8 is a flow diagram illustrating the configuration of thecontroller 514 according to an embodiment of the present invention. Inresponse to receiving a reset signal, at 800, the controller 514 resetsthe controller 514 and/or driver 512 components. At 802, the controller514 produces a control signal to the driver 512 to initiate energizingthe LED 508 (i.e., providing the driver 512 output current to the LED508). At 804, the controller 514 senses the LED voltage(V_(led)). At806, the controller 514 is configured to compare the LED voltage(V_(led)) to a predetermined value (e.g., value stored in the storagemedium). In particular, the controller 514 is configured to determinewhether the LED voltage (V_(led)) is greater than a maximum voltagevalue (V_(led max)). If the controller 514 determines that the LEDvoltage (V_(led)) is greater than the maximum voltage value(V_(led max)), the controller 514 is configured to operate in a shutdown mode at 808. In the shut down mode, the controller 514, responsiveto receiving a power on reset signal, resumes normal operation asdiscussed above at 800. If the controller 514 determines the LED voltage(V_(led)) is less than the maximum voltage value (V_(ledmax)) thecontroller 514 compares at 810 the LED voltage (V_(led)) to apredetermined value (e.g., value stored in the storage medium). Inparticular, the controller 514 is configured to determine whether theLED voltage value (V_(led)) is less than a maximum power value dividedby a maximum current value (P_(led max)/I_(ledmax)). If the controller514 determines that the LED voltage (V_(led)) value is less than themaximum power value divided by the maximum current value(P_(led max)/I_(ledmax)), the controller 514 is configured to operate ina first operating mode. In the first operating mode the controller 514sets the control signal value to correspond to the maximum current value(I_(ledmax)) at 812, transmits the control signal value at 816 andsenses the LED voltage (V_(led)) as previously discussed above at 804.Thus, in the first operating mode the controller 514 controls the driver512 to provide a substantially constant driver output current (I_(led))to the LED 508. If the controller 514 determines that the LED voltagevalue (V_(led)) is not less than the maximum power value divided by themaximum current value (P_(led max)/I_(ledmax)), the controller 514 isconfigured to operate in a second operating mode. In the secondoperating mode, the controller 514 at 814 determines a value for thedriver current as a function of the LED voltage value (V_(led)). Forexample, the controller 514 may reference a look up table or an equation(e.g., P_(led max)/V_(led)) to determine the value for the driver outputcurrent (I_(led)) for operating the LED at the maximum power value(P_(led max)). The controller 514 transmits the control signal value 516having a value corresponding to the determined driver output currentvalue (I_(led)) at 816, and senses the LED voltage (V_(led)) aspreviously discussed above at 804. Thus, in the second operating mode,the controller 514 controls the driver 512 to vary the driver outputcurrent (I_(led)) to the LED 508 according a constant power value.

The order of execution or performance of the operations in embodimentsof the invention illustrated and described herein is not essential,unless otherwise specified. That is, the operations may be performed inany order, unless otherwise specified, and embodiments of the inventionmay include additional or fewer operations than those disclosed herein.For example, it is contemplated that executing or performing aparticular operation before, contemporaneously with, or after anotheroperation is within the scope of aspects of the invention.

Embodiments of the invention may be implemented with computer-executableinstructions. The computer-executable instructions may be organized intoone or more computer-executable components or modules. Aspects of theinvention may be implemented with any number and organization of suchcomponents or modules. For example, aspects of the invention are notlimited to the specific computer-executable instructions or the specificcomponents or modules illustrated in the figures and described herein.Other embodiments of the invention may include differentcomputer-executable instructions or components having more or lessfunctionality than illustrated and described herein.

When introducing elements of aspects of the invention or the embodimentsthereof, the articles “a,” “an,” “the,” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Having described aspects of the invention in detail, it will be apparentthat modifications and variations are possible without departing fromthe scope of aspects of the invention as defined in the appended claims.As various changes could be made in the above constructions, products,and methods without departing from the scope of aspects of theinvention, it is intended that all matter contained in the abovedescription and shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

1. An apparatus used with a voltage source for energizing a lightemitting diode (LED), said apparatus comprising: a driver receiving aninput voltage from the voltage source and a control signal and providinga driver output current to the LED based on the control signal, saiddriver having a boost buck topology, said LED having an LED voltage as afunction of said driver output current and having an LED power as afunction of said LED voltage and said driver output current; and acontroller determining the LED voltage and producing the control signalprovided to the driver as a function of the LED voltage, said controlsignal corresponding to a constant driver output current when the LEDvoltage is less than a predetermined voltage value, said control signalcorresponding to a varying driver output current for substantiallymaintaining the LED power at a constant predetermined power value whenthe LED voltage is greater than the predetermined voltage value.
 2. Theapparatus of claim 1 wherein the driver comprises a voltage controlledcurrent source.
 3. The apparatus of claim 1 wherein the driver comprisesa switching component having a switching frequency for providing thedriver output current to the LED based on the control signal.
 4. Theapparatus of claim 1 wherein the driver receives the driver outputcurrent and provides the driver output current to the LED based on thecontrol signal and the driver output current.
 5. The apparatus of claim1 wherein the controller comprises storage medium for storing data fordefining the control signal provided to the driver as a function of theLED voltage, said stored data comprising values for the LED voltagemapped to values of the driver output current.
 6. The apparatus of claim1 wherein the constant driver output current has a maximum current valueand the predetermined power value is a maximum power value and whereinthe predetermined voltage value is the maximum power value divided bythe maximum current value.
 7. The apparatus of claim 1 wherein thecontrol signal is a voltage signal.
 8. An apparatus used with a voltagesource for controlling an input parameter for a light emitting diode(LED), said apparatus comprising: a driver receiving an input voltagefrom the voltage source and providing a driver output current to theLED, said driver having a boost buck topology, said LED having an LEDvoltage as a function of said driver output current and having an LEDpower as a function of said LED voltage and said driver output current;and a controller for receiving the LED voltage and operating in a firstmode and a second mode as a function of said LED voltage, said firstmode controlling the driver to provide a substantially constant driveroutput current to the LED, said second mode controlling the driver tovary the driver output current to the LED according to a constant powervalue.
 9. The apparatus of claim 8 further comprising the LED.
 10. Theapparatus of claim 8 wherein the controller operates in a first mode, asecond mode, and a third mode as a function of said voltage, said firstmode controlling the driver to provide a substantially constant driveroutput current to the LED, said second mode controlling the driver tovary the driver output current to the LED according to a constant powervalue, said third mode controlling the driver cease providing a driveroutput current to the LED.
 11. The apparatus of claim 8 furthercomprising a means for electrical communication from the controller tothe driver, said controller controlling said driver via said means forelectrical communication.
 12. The apparatus of claim 8 wherein thedriver comprises a switching component having a switching frequency forproviding the driver output current to the LED based on the controlsignal.
 13. The apparatus of claim 8 wherein the driver receives thedriver output current and subsequently provides the driver outputcurrent to the LED based on the control signal and the driver outputcurrent.
 14. The apparatus of claim 8 wherein the controller comprises astorage medium for storing data for defining the control signal providedto the driver as a function of the LED voltage, said stored datacomprising values for the LED voltage mapped to values of the driveroutput current.
 15. The apparatus of claim 8 wherein the controlleroperates in a first mode when said LED voltage is greater than apredetermined voltage value and said controller operates in a secondmode when said LED voltage is less than the predetermined voltage value.16. The apparatus of claim 15 wherein the driver output current is amaximum current value and the constant power value is a maximum powervalue and wherein the predetermined voltage value is the maximum powervalue divided by the maximum current value.
 17. A method for controllingan electrical energy parameter provided to a light emitting diode (LED),said method comprising: sensing by a driver a first electrical energyparameter of the LED; sensing by a controller a second electrical energyparameter of the LED, said LED having an LED power as a function of thefirst electrical energy parameter and said second electrical energyparameter; determining by the controller whether the second electricalenergy parameter is greater than a predetermined value; generating bythe controller a control signal based on said determining, said controlsignal specifying a value for the first electrical parameter to maintainthe first electrical energy parameter as constant when the secondelectrical energy parameter is not greater than the predeterminedreference electrical energy value, said control signal specifying avalue for the first electrical parameter to maintain the LED power asconstant when the second electrical energy parameter is greater than thepredetermined reference electrical energy value; and controlling by thedriver the first electrical energy parameter provided to the LED basedon the first electrical energy parameter and the control signalgenerated by the controller.
 18. The method of claim 18 wherein thefirst electrical energy parameter is current.
 19. The method of claim 18wherein the second electrical energy parameter is voltage.
 20. Anapparatus used with a voltage source, said apparatus comprising: a lightemitting diode (LED); a driver receiving an input voltage from thevoltage source and a control signal and providing a driver outputcurrent to the LED based on the control signal, said driver having aboost buck topology, said LED having an LED voltage as a function ofsaid driver output current and having an LED power as a function of saidLED voltage and said driver output current; and a controller forreceiving the LED voltage and producing the control signal provided tothe driver as a function of the LED voltage, said control signalrepresentative of constant values for the driver output current when theLED voltage is less than a predetermined voltage value, said controlsignal representative of values for the driver output current varying tosubstantially maintain the LED power at a constant predetermined powervalue wherein the LED voltage is greater than the predetermined voltagevalue.